That's unfair, i'm not trying to put anybody down or advocating treating anyone unpleasantly, i'm just pointing out that a cashier is actually paid to pay attention to the customer. If they ignore you, to the detriment of the transaction, that's a deriliction of their professional duty to you, it's not at all the same as the customer not paying attention to them. It was him drawing a direct equivalency I was complaining at.
Certainly I advocate treating everybody with respect, my point here is that nobody has provided a reason why
it is bad manners to be on the phone while being served, as long as your actions are not slowing the process down. "It's common courtesy" and "I was raised to be polite" are not convincing arguments, they are just appeals to tradition.
The customer has eyes as well as ears, they can see if the cashier requires their attention, if they ignored the cashier when they needed their input that
would be rude. But I don't see who or what is served by the customer standing there paying attention to them scanning shopping, if they have something better to be doing.
Is it also discourteous to talk to the person standing next to you? Or is it only when your conversational partner is disembodied that it's rude not to be paying full attention to the cashier?